Which form of ammonia is diffusible?

Ammonia exists in two primary forms in solution: the positively charged ammonium ion (NH4+) and the neutral ammonia molecule (NH3). The diffusible form of ammonia is NH3. This is primarily because NH3 is uncharged, which allows it to easily cross cellular membranes through passive diffusion.

The presence of the ammonium ion (NH4+) means that it carries a positive charge, which significantly hampers its ability to diffuse across cell membranes, as charged particles typically require specific transport mechanisms to facilitate their movement. The negatively charged ammonide ion (NH2-) is also unable to diffuse readily due to its charge and instability under physiological conditions.

In summary, the diffusible form of ammonia is NH3 due to its neutral charge, enabling it to cross membranes freely and participate in various biochemical processes, including those involving neurotransmission and detoxification in the liver. This property of NH3 is critical for understanding how ammonia is metabolized and regulated in the body.